The present invention relates to an x-ray system and method for identifying material within an obscuring enclosure, and more particularly to a system and method using coherently scattered penetrating radiation for discriminating target materials.
The angular distribution of x-ray radiation scattered from a material when the radiation incident on the material is substantially monochromatic provides a well-established method for identifying the scattering material. The basis of the identifying characteristics of the scattered radiation is coherent x-ray scattering from the crystal planes of the bulk material. The well-known Bragg equation governs this so-called wavelength dispersive spectroscopy:
sinxcex8=nxcex/(2d), (1)
where d is the spacing between crystal planes, xcex8 is the scattering angle, n is the order of scattering and xcex is the wave length of the radiation. Practitioners typically use low energy x-rays for these measurements, for example, the 8 keV (1.5 xc3x85) x-rays from copper produce strong Bragg peaks at large, easily measured, scattering angles.
However, the identification of material in the interior of large containers typically employs radiation of higher energy. In particular, for luggage brought on board aircraft, typical x-ray energies are at least 75 keV, corresponding to a wavelength of ⅙th of an Angstrom. At this energy, the first Bragg peak (the closest to xcex8=0xc2x0) will then be at a very small angle, typically in the range of a few degrees, making wavelength dispersive spectroscopy extremely difficult.
A more practical approach for the use of coherent scattering at higher energies, suggested by G. Harding and J. Kosanetzky, xe2x80x9cScattered X-Ray Beam Non-Destructive Testing,xe2x80x9d in Nuclear Instruments and Methods (1989), is to use energy dispersive spectroscopy. In energy dispersive spectroscopy, a polychromatic beam of high energy x-rays is sent through the container and the energy distribution at a fixed scattering angle of a few degrees is used to identify the object. The governing equation is the same as Eqn. 1, written to emphasize the energy dependence:                               E          =                                    6.2                              d                ⁢                                  xe2x80x83                                ⁢                sin                ⁢                                  xe2x80x83                                ⁢                θ                                      ≅                          6.2                              d                ⁢                                  xe2x80x83                                ⁢                θ                                                    ,                            (        2        )            
where d is the crystalline spacing in Angstroms, xcex8 is the scattering angle in radians, and E is the x-ray energy in keV. Thus, for example, an x-ray of 100 keV will be Bragg scattered through an angle of about 2xc2x0 by a crystalline substance with spacings of about 2 xc3x85.
Bragg-scattering inspection systems under current development seek to examine the entire volume of every piece of luggage that enters an aircraft. The hardware to carry out this daunting task is complex and expensive, and are at least 2 orders of magnitude too slow to be effective as a screener at an airport terminal.
Additionally, since the Bragg scattering angles are so low (typically 2xc2x0-30xc2x0), the collimation requirements on the detector are stringent if a particular volume along the x-ray path into the interrogated volume is to be discriminated. The strict requirement on the collimation of the coherent-scatter detector can be quantified by noting that an uncertainty in the angle results in an uncertainty in the measured energy. Differentiating Equation (2) gives the necessary relation:                                           Δ            ⁢                          xe2x80x83                        ⁢            E                    E                ≅                  -                                                    Δ                ⁢                                  xe2x80x83                                ⁢                θ                            θ                        .                                              (        3        )            
To obtain a full-width energy resolution of xcex94E/E=5%, the angular uncertainty xcex94xcex8/xcex8 must be kept to 5%. (A 5% uncertainty is typical of the maximum uncertainty that can be tolerated if the coherent scatter method is to effectively discriminate between different types of materials.) The collimation must therefore be good enough to limit the acceptance angle to 2xc2x0 with an accuracy of 5%, a difficult requirement.
The small scattering angles with their tight uncertainty requirements severely restrict the length along the beam that can be inspected by a single coherent-scatter detector, typically to no more than 3 cm. If the position along the beam path of a suspect volume of an inspected enclosure is unknown, then it becomes necessary to make 5 to 10 separate measurements (or, alternatively, to provide the same number of carefully collimated detector elements) to inspect all the voxels along a given beam path. In one case, inspection times are increased, and in the other, the cost of the system is impacted substantially.
In accordance with one aspect of the invention, in a preferred embodiment, there is provided an inspection system for inspecting an enclosure. The system has a source for producing a pencil beam of penetrating radiation and a substantially inertia-free scanner for scanning the beam through successive positions with respect to the enclosure. The system has at least one detector for generating a signal based on at least one of penetrating radiation transmitted through the enclosure and penetrating radiation scattered by the enclosure, a processor for identifying positions warranting scrutiny on the basis of the signal from the at least one detector and a set of specified conditions, and at least one Bragg detector for determining a spectrum of coherent scattering from an identified volume within the enclosure. In accordance with alternate embodiments of the invention, the at least one Bragg detector may be a multi-element solid-state detector. The system may also include a fiducial material disposed in the beam posterior to the enclosure with respect to the source for correcting spectral distortions due to wavelength-dependent absorption of the beam of penetrating radiation.
In a preferred embodiment, the scanner may have a beam control arrangement for scanning a position of an electron beam with respect to an x-ray emissive anode and a perforated absorbing shield for permitting emission of penetrating radiation at a single emission angle determined by the position of the electron beam. The beam control arrangement may include at least one of a magnetic control yoke and an array of electrical deflector plates. In accordance with other embodiments of the invention, the scanner may include a mechanically positionable aperture, and the inspection system may also include a translator for positioning the at least one Bragg detector in response to identification by the processor of positions warranting scrutiny.
In accordance with one aspect of the invention, in one of its embodiments, there is provided an inspection system for inspecting an enclosure. The system has a source for producing a beam of penetrating radiation and a scanner for scanning the beam through successive positions with respect to the enclosure. The inspection system has a set of detectors disposed along a direction substantially parallel to the beam, the set of detectors generating a sidescatter signal based on penetrating radiation sidescattered by the object, and a controller for identifying a position of a suspected object based at least in part upon the sidescatter signal. Additionally, the inspection system has at least one Bragg detector for determining a spectrum of coherent scattering from the position of the suspected object.
In accordance with alternate embodiments of the invention, the beam of penetrating radiation may have a specified beam profile, more particularly, that of a pencil beam. The beam of penetrating radiation may be an x-ray beam. Each Bragg detector may be a multi-element solid-state detector and may be energy-dispersive. The inspection system may also include a fiducial material disposed in the beam posterior to the enclosure with respect to the source for correcting spectral distortions due to wavelength-dependent absorption of the beam of penetrating radiation.
In accordance with further embodiments of the invention, an inspection system is provided for characterizing the nature and three-dimensional position of an object contained within an enclosure. In addition to a source for producing a beam of penetrating radiation and a scanner for scanning the beam through successive positions with respect to the enclosure, the system has a set of detectors disposed along a direction substantially parallel to the beam, the set of detectors generating a sidescatter signal based on penetrating radiation sidescattered by the object. The system also has a controller for identifying a position of a suspected object based at least in part upon the sidescatter signal and a plurality of Bragg detectors disposed along a direction substantially transverse to the beam for measuring an angular distribution of coherently scattered penetrating radiation.
In accordance with a further aspect of the present invention in one of its embodiments, there is provided a method for inspecting an enclosure. The method has the steps of:
a. producing a pencil beam of penetrating radiation;
b. scanning the beam through successive positions with respect to the enclosure;
c. detecting with an x-ray detector at least one of penetrating radiation transmitted through the enclosure and penetrating radiation scattered by the enclosure;
d. identifying positions warranting scrutiny on the detected penetrating radiation; and
e. determining a spectrum of coherent scattering from an identified volume within the enclosure during the course of a single pass of the enclosure past the beam.
The method may also have a step of determining a spectrum of coherent scattering from a reference material disposed within the beam at a position posterior to the enclosure with respect to the source. Additionally, the method may have the steps of determining spectra of coherent scattering from a plurality of volumes within the enclosure and including positioning a reference material between each of the plurality of volumes and the x-ray detector.